The invention concerns an arrangement for transmitting the torque between the two rotors of a two-stage rotary vane pump.
In two-stage rotary vane pumps, for reasons of cost the high vacuum stage is arranged coaxially with the atmospheric stage. The rotor of the atmospheric stage, driven by an electric motor, transfers the necessary torque to the rotor of the high vacuum stage. For the purposes of the coupling arrangement according to the invention, however, the two rotors may be interchanged.
It is known from the prior art that the rotors could be arranged on a single shaft without a coupling. In this case, however, vane slits extending radially right through the rotor are not possible, so that the diameter of the rotors must be made substantially larger, and this disadvantageously affects the friction losses and the size of the whole pump.
Moreover, rotors for these purposes are known which are not arranged on a single shaft, but are provided with individual bearings. In this case, the rotors have on their opposed faces grooves and bars which inter-engage to form a coupling.
This form of coupling has only one radial degree of freedom, parallel to the groove and bar. Since neither the groove nor the bar can be made absolutely symmetrical relative to the axis, the bearing pins are radially displaced by the sum of the two errors. Due to the oscillating mechanical displacement of the two bearing pins, satisfactory hydrodynamic support of the rotors is not possible. Moreover, since an effective lubrication film cannot form, damage to the bearings results. Enlargement of the play between the groove and bar does not solve the problem, since in transmitting the torque, the bar and groove engage one another diagonally, and the play provided disappears. The displacement of the bearing pins remains. Further, this solution runs counter to the requirement for noise reduction.
Commercially available couplings can be used for connecting the two rotor parts, and these at least partly reduce the above-mentioned problems and their resulting damage. These, however, use up a relatively large space and thus disadvantageously increase the size of the whole pump.